The present invention relates to particle separation, particularly to collecting particles for subsequent characterization or analysis, and more particularly to an opposed-flow virtual cyclone which accurately collects, classifies, and concentrates (enriches) particles in a specific size range.
Particle separation, characterization, and/or analysis has involved continuous research and development for many decades. However, more recently, substantial interest has been directed to collection and analysis of air-borne particles, referred to as aerosol collectors. As used here, the term aerosol refers to liquid or solid particles which are suspended in a gas (e.g., air). The particles may be anthropogenic (such as smog, fly-ash, or smoke) or naturally occurring (such as pollens, dust, or mists). One problem facing the aerosol field is that of collecting the particles for subsequent characterization or analysis. Although some characterization of these air-borne particles can be performed in situ, in many cases it is essential to deposit these particles onto a solid substrate, or to inject them into a liquid for the purpose of detailed physical or chemical analysis. Consequently, a large number of aerosol collectors have been developed in the past, including devices such as jet impingers, jet impactors, and filters.
In many cases it is desirable to sort the particles according to size as part of the collection process, such as to determine the aerosol size distribution (mass of suspended particles as a function of particle size per volume of gas) or to identify any size-related dependencies of particle chemical characteristics. A large number of aerosol collectors with size-classification capabilities appear in the prior art, including cascade inertial impactors, cyclones, and inertial spectrometers. The aerosol collection task is further complicated when particles in a specific size interval or range are of interest, especially when the concentration of these particles is very low (where particle concentration is given by the number of particles per unit volume of gas).
Of particular interest is the example of bioaerosols, which includes air-borne pollens, viruses, or bacteria. Bioaerosols can result from natural processes (pollen releases by plants), or from human activities by inadvertent (e.g., in operating rooms, communicable diseases) or intentional (agricultural or battlefield) releases. The concentration of bioaerosols can be quite low, and the challenge in their analysis is to separate them from a potentially high concentration of background aerosol. For many aerosol collection problems, including bioaerosols, the ideal collector would be one which accurately collects, classifies, and concentrates (enriches) particles in a specific size range.
One prior approach to aerosol enrichment is the aerodynamic lens, schematically illustrated in FIG. 1, which concentrates particles along the centerline of an axisymmetric geometry through a series of flow contractions and enlargements. See Peng et al., Aerosol Sci. Technol. 22:293-313 and 314-325, 1995. After each contraction, particles are moved closer to the centerline if their aerodynamic sizes are less than a critical size, while particles larger than the critical size more further from the centerline. Through a careful design of a series of lenses, particle enrichment within a specific size range can be achieved. However, there are several drawbacks to the aerodynamic lens, including: 1) complexity in design, construction, and alignment of a multi-lens system, 2) difficulties in getting high enrichments over a wide size range, and 3) particle deposition and build-up on walls of the lens.
Another prior means of achieving particle enrichment is the virtual impactor, schematically shown in FIG. 2, and which generally consists of an axisymrnetric jet impinging on a normal plate which has a small hole (perhaps leading to a cavity below) in it located at the jet centerline. See Marple et al., Environ. Sci. Technol., 14:976-985, 1980. If flow through the hole is restricted, then the region behind the hole becomes a stagnation zone, which acts as a "virtual surface." Thus, the impinging jet is deflected by the plate (and the virtual surface at its center) and flows radially outward. Because of their inertia, particles cannot make the turn and are impacted into the virtual surface, leading to particle enrichment in the cavity below. By careful design of a series of vertical impactor stages, particle enrichment in a specific size range can be achieved. However, the virtual impactor suffers many of the same drawbacks as the aerodynamic lens: 1) complexity in design, construction, and alignment, 2) difficulties in getting high enrichments over a wide size range, and 3) particle deposition and build-up on walls.
Recently, a virtual cyclone, schematically illustrated in FIGS. 3A and 3B, was developed as a means of separating particles form a main flow and concentrating them in an adjacent recirculating chamber. See Torczynski and Rader, "The Virtual Cyclone: A Device for Nonimpact Particle Separation", Aerosol Sci. Technol., 26:560-573, 1997. In the virtual cyclone, the main particle-laden flow follows a wall that curves away from the original flow direction. Although a wall forms the inner boundary of the main flow, its outer boundary is formed by an adjacent flow, often a confined recirculating flow, into which particles are transferred by centrifugal action. Thus, in the virtual cyclone, particles are separated from the main flow by crossing a dividing streamline that separates the main flow stream from an adjacent flow stream. If a confined recirculating chamber geometry is used, particle concentrations in the recirculating region can be greatly increased relative to the main stream. A primary advantage of the virtual cyclone is that it accomplishes inertial separation in such a way as to greatly reduce particle deposition on walls.
However, experiments have shown that turbulent mixing produced by shear-layer roll-up can limit particle-concentration enhancement at high flow Reynolds numbers. In addition, removing particles from the recirculating chamber for analysis presents some challenge in the virtual cyclone geometry. Thus, the need persists for a device which accurately collects, classifies, and concentrates (enriches) particles of interest.
The present invention provides a solution to the above-mentioned need by providing an opposed-flow virtual cyclone, which is a variation on the above-referenced virtual cyclone in that it preserves its inherent advantages (no-impact particle separation in a simple geometry), while providing a more robust design for concentrating particles in a flow-through type system. In simplest terms, the opposed-flow virtual cyclone of the present invention consists of two geometrically similar virtual cyclones arranged such that their inlet jets are inwardly directed and symmetrically opposed relative to a plane of symmetry located midway between the two inlets. As in the virtual cyclone, each inlet jet will follow the adjacent curved wall as it turns away and that particles will be transferred away from the curved wall and towards the symmetry plane by centrifugal action. After turning, the two jets merge smoothly along the symmetry line and flow parallel to it. The particles are transferred from the main flows, across the dividing streamline, and into the central recirculating region, where particle concentrations become greatly increased relative to the main flow. Thus, the present invention provides a device which is able to accurately collect, classify, and concentrate (enrich) particles of interest.